Conformational Sensors and Domain Swapping Reveal Structural and Functional Differences between β-Arrestin Isoforms

Molecular mechanism of distinct chemokine engagement and functional divergence of the human Duffy antigen receptor

The Duffy antigen receptor is a seven-transmembrane (7TM) protein expressed primarily at the surface of red blood cells and displays strikingly promiscuous binding to multiple inflammatory and homeostatic chemokines. It serves as the basis of the Duffy blood group system in humans and also acts as the primary attachment site for malarial parasite Plasmodium vivax and pore-forming toxins secreted by Staphylococcus aureus. Here, we comprehensively profile transducer coupling of this receptor, discover potential non-canonical signaling pathways, and determine the cryoelectron microscopy (cryo-EM) structure in complex with the chemokine CCL7. The structure reveals a distinct binding mode of chemokines, as reflected by relatively superficial binding and a partially formed orthosteric binding pocket. We also observe a dramatic shortening of TM5 and 6 on the intracellular side, which precludes the formation of the docking site for canonical signal transducers, thereby providing a possible explanation for the distinct pharmacological and functional phenotype of this receptor.

Mapping the interaction site for β-arrestin-2 in the prokineticin 2 receptor

G protein-coupled receptors (GPCRs) are a family of cell membrane receptors that couple and activate heterotrimeric G proteins and their associated intracellular signalling processes after ligand binding. Although the carboxyl terminal of the receptors is essential for this action, it can also serve as a docking site for regulatory proteins such as the β-arrestins. Prokineticin receptors (PKR1 and PKR2) are a new class of GPCRs that are able to activate different classes of G proteins and form complexes with β-arrestins after activation by the endogenous agonists PK2. The aim of this work was to define the molecular determinants within PKR2 that are required for β-arrestin-2 binding and to investigate the role of β-arrestin-2 in the signalling pathways induced by PKR2 activation. Our data show that PKR2 binds constitutively to β-arrestin-2 and that this process occurs through the core region of the receptor without being affected by the carboxy-terminal region. Indeed, a PKR2 mutant lacking the carboxy-terminal amino acids retains the ability to bind constitutively to β-arrestin-2, whereas a mutant lacking the third intracellular loop does not. Overall, our data suggest that the C-terminus of PKR2 is critical for the stability of the β-arrestin-2-receptor complex in the presence of PK2 ligand. This leads to the β-arrestin-2 conformational change required to initiate intracellular signalling that ultimately leads to ERK phosphorylation and activation.

Structure-guided engineering of biased-agonism in the human niacin receptor via single amino acid substitution

The Hydroxycarboxylic acid receptor 2 (HCA2), also known as the niacin receptor or GPR109A, is a prototypical GPCR that plays a central role in the inhibition of lipolytic and atherogenic activities. Its activation also results in vasodilation that is linked to the side-effect of flushing associated with dyslipidemia drugs such as niacin. GPR109A continues to be a target for developing potential therapeutics in dyslipidemia with minimized flushing response. Here, we present cryo-EM structures of the GPR109A in complex with dyslipidemia drugs, niacin or acipimox, non-flushing agonists, MK6892 or GSK256073, and recently approved psoriasis drug, monomethyl fumarate (MMF). These structures elucidate the binding mechanism of agonists, molecular basis of receptor activation, and insights into biased signaling elicited by some of the agonists. The structural framework also allows us to engineer receptor mutants that exhibit G-protein signaling bias, and therefore, our study may help in structure-guided drug discovery efforts targeting this receptor.

Molecular insights into atypical modes of β-arrestin interaction with seven transmembrane receptors

β-arrestins (βarrs) are multifunctional proteins involved in signaling and regulation of seven transmembrane receptors (7TMRs), and their interaction is driven primarily by agonist-induced receptor activation and phosphorylation. Here, we present seven cryo-electron microscopy structures of βarrs either in the basal state, activated by the muscarinic receptor subtype 2 (M2R) through its third intracellular loop, or activated by the βarr-biased decoy D6 receptor (D6R). Combined with biochemical, cellular, and biophysical experiments, these structural snapshots allow the visualization of atypical engagement of βarrs with 7TMRs and also reveal a structural transition in the carboxyl terminus of βarr2 from a β strand to an α helix upon activation by D6R. Our study provides previously unanticipated molecular insights into the structural and functional diversity encoded in 7TMR-βarr complexes with direct implications for exploring novel therapeutic avenues.

Enabling technology and core theory of synthetic biology

Synthetic biology provides a new paradigm for life science research ("build to learn") and opens the future journey of biotechnology ("build to use"). Here, we discuss advances of various principles and technologies in the mainstream of the enabling technology of synthetic biology, including synthesis and assembly of a genome, DNA storage, gene editing, molecular evolution and de novo design of function proteins, cell and gene circuit engineering, cell-free synthetic biology, artificial intelligence (AI)-aided synthetic biology, as well as biofoundries. We also introduce the concept of quantitative synthetic biology, which is guiding synthetic biology towards increased accuracy and predictability or the real rational design. We conclude that synthetic biology will establish its disciplinary system with the iterative development of enabling technologies and the maturity of the core theory.

Structural snapshots uncover a key phosphorylation motif in GPCRs driving β-arrestin activation

Agonist-induced GPCR phosphorylation is a key determinant for the binding and activation of β-arrestins (βarrs). However, it is not entirely clear how different GPCRs harboring divergent phosphorylation patterns impart converging active conformation on βarrs leading to broadly conserved functional responses such as desensitization, endocytosis, and signaling. Here, we present multiple cryo-EM structures of activated βarrs in complex with distinct phosphorylation patterns derived from the carboxyl terminus of different GPCRs. These structures help identify a P-X-P-P type phosphorylation motif in GPCRs that interacts with a spatially organized K-K-R-R-K-K sequence in the N-domain of βarrs. Sequence analysis of the human GPCRome reveals the presence of this phosphorylation pattern in a large number of receptors, and its contribution in βarr activation is demonstrated by targeted mutagenesis experiments combined with an intrabody-based conformational sensor. Taken together, our findings provide important structural insights into the ability of distinct GPCRs to activate βarrs through a significantly conserved mechanism.

Plasma membrane preassociation drives β-arrestin coupling to receptors and activation

β-arrestin plays a key role in G protein-coupled receptor (GPCR) signaling and desensitization. Despite recent structural advances, the mechanisms that govern receptor-β-arrestin interactions at the plasma membrane of living cells remain elusive. Here, we combine single-molecule microscopy with molecular dynamics simulations to dissect the complex sequence of events involved in β-arrestin interactions with both receptors and the lipid bilayer. Unexpectedly, our results reveal that β-arrestin spontaneously inserts into the lipid bilayer and transiently interacts with receptors via lateral diffusion on the plasma membrane. Moreover, they indicate that, following receptor interaction, the plasma membrane stabilizes β-arrestin in a longer-lived, membrane-bound state, allowing it to diffuse to clathrin-coated pits separately from the activating receptor. These results expand our current understanding of β-arrestin function at the plasma membrane, revealing a critical role for β-arrestin preassociation with the lipid bilayer in facilitating its interactions with receptors and subsequent activation.

The Ubiquitination Status of the Glucagon Receptor determines Signal Bias

The pancreatic hormone glucagon activates the glucagon receptor (GCGR), a class B seven-transmembrane G protein-coupled receptor (GPCR) that couples to the stimulatory heterotrimeric Gs protein and provokes protein kinase A-dependent signaling cascades vital to hepatic glucose metabolism and islet insulin secretion. Glucagon-stimulation also initiates recruitment of the endocytic adaptors, β-arrestin1 and β-arrestin2, which regulate desensitization and internalization of the GCGR. Unlike many other GPCRs, the GCGR expressed at the plasma membrane is constitutively ubiquitinated and upon agonist-activation, internalized GCGRs are deubiquitinated at early endosomes and recycled via Rab4-containing vesicles. Herein we report a novel link between the ubiquitination status and signal transduction mechanism of the GCGR. In the deubiquitinated state, coupling of the GCGR to Gs is diminished, while binding to β-arrestin is enhanced with signaling biased to a β-arrestin1-dependent p38 mitogen activated protein kinase (MAPK) pathway. This ubiquitin-dependent signaling bias arises through the modification of lysine333 (K333) on the cytoplasmic face of transmembrane helix V. Compared with the GCGR-WT, the mutant GCGR-K333R has impaired ubiquitination, diminished G protein coupling and protein kinase A signaling, but unimpaired potentiation of glucose-stimulated-insulin secretion in response to agonist-stimulation, which involves p38 MAPK signaling. Both WT and GCGR-K333R promote the formation of glucagon-induced β-arrestin1-dependent p38 signaling scaffold that requires canonical upstream MAPK-Kinase3, but is independent of Gs, Gi and β-arrestin2. Thus ubiquitination/deubiquitination at K333 in the GCGR defines the activation of distinct transducers with the potential to influence various facets of glucagon signaling in health and disease.

Comparative Study of Elabela and Apelin on Apelin Receptor Activation Through β-Arrestin Recruitment

Apelin receptor (APJ) ligands elabela (ELA) and apelin have divergent distributions and function differently in vitro and in vivo. Whether differences exist in their capacity of recruitment of β-arrestins (ARRBs) to APJ remains unknown. The aim of the current study was to investigate the different effects of ELA and apelin on the interaction between APJ and ARRBs in live cells by NanoBiT®. NanoBiT® system is a new technology for studying protein-protein interaction in real-time in live cells, based on the emission of luminescence when two split components of NanoLuc luciferase, large Bit (LgBit) and small Bit (SmBit), complement each other to form an enzymatically active entity. We tagged the APJ and ARRBs with LgBit or SmBit and then evaluated their interactions in transiently transfected HEK293T cells, and determined the signal strength yielded as a result of the interaction. We also investigated the concentration-dependent response of the APJ-ARRB interaction in response to ELA and apelin. Finally, we assessed the effect of F13A, an APJ antagonist which is structurally very similar to apelin-13, on ELA- and apelin-mediated APJ-ARRB interactions. The NanoLuc® luciferase signal was highest in the pair of APJ-LgBit with SmBit-ARRB1 or SmBit-ARRB2. NanoLuc® luciferase signal increased in a concentration-dependent manner from 0.1 nM to 10 μM in response to ELA or apelin. Interestingly, ELA elicited weaker APJ-ARRB interaction signals than apelin. Pre-treatment with F13A potently reduced the APJ-ARRB interaction in response to both ELA and apelin. Our results demonstrated that both ELA and apelin promoted the interaction of APJ and ARRBs in a concentration-dependent manner, and ELA is less efficacious than apelin in inducing the recruitment of ARRBs to APJ, providing a biased functional aspect of ELA vs. apelin at the receptor signaling level. Additionally, ELA and apelin may share the same binding site(s) or pocket(s) at the APJ level.

β-arrestin1 and 2 exhibit distinct phosphorylation-dependent conformations when coupling to the same GPCR in living cells

β-arrestins mediate regulatory processes for over 800 different G protein-coupled receptors (GPCRs) by adopting specific conformations that result from the geometry of the GPCR–β-arrestin complex. However, whether β-arrestin1 and 2 respond differently for binding to the same GPCR is still unknown. Employing GRK knockout cells and β-arrestins lacking the finger-loop-region, we show that the two isoforms prefer to associate with the active parathyroid hormone 1 receptor (PTH1R) in different complex configurations (“hanging” and “core”). Furthermore, the utilisation of advanced NanoLuc/FlAsH-based biosensors reveals distinct conformational signatures of β-arrestin1 and 2 when bound to active PTH1R (P-R*). Moreover, we assess β-arrestin conformational changes that are induced specifically by proximal and distal C-terminal phosphorylation and in the absence of GPCR kinases (GRKs) (R*). Here, we show differences between conformational changes that are induced by P-R* or R* receptor states and further disclose the impact of site-specific GPCR phosphorylation on arrestin-coupling and function.

Here the authors present improved intramolecular sensors for β-arrestin2 and 1, which enable assessment of conformational changes of both isoforms in living cells. These reveal that the same GPCR induces differential conformational rearrangements that determine the functional diversity between the two β-arrestins.

G protein-coupled receptor signaling: transducers and effectors

G protein-coupled receptors (GPCRs) are of considerable interest due to their importance in a wide range of physiological functions and in a large number of Food and Drug Administration (FDA)-approved drugs as therapeutic entities. With continued study of their function and mechanism of action, there is a greater understanding of how effector molecules interact with a receptor to initiate downstream effector signaling. This review aims to explore the signaling pathways, dynamic structures, and physiological relevance in the cardiovascular system of the three most important GPCR signaling effectors: heterotrimeric G proteins, GPCR kinases (GRKs), and β-arrestins. We will first summarize their prominent roles in GPCR pharmacology before transitioning into less well-explored areas. As new technologies are developed and applied to studying GPCR structure and their downstream effectors, there is increasing appreciation for the elegance of the regulatory mechanisms that mediate intracellular signaling and function.

Allosteric modulation of GPCR-induced β-arrestin trafficking and signaling by a synthetic intrabody

Agonist-induced phosphorylation of G protein-coupled receptors (GPCRs) is a primary determinant of β-arrestin (βarr) recruitment and trafficking. For several GPCRs such as the vasopressin receptor subtype 2 (V₂R), agonist-stimulation first drives the translocation of βarrs to the plasma membrane, followed by endosomal trafficking, which is generally considered to be orchestrated by multiple phosphorylation sites. We have previously shown that mutation of a single phosphorylation site in the V₂R (i.e., V₂R^T360A) results in near-complete loss of βarr translocation to endosomes despite robust recruitment to the plasma membrane, and compromised ERK1/2 activation. Here, we discover that a synthetic intrabody (Ib30), which selectively recognizes activated βarr1, efficiently rescues the endosomal trafficking of βarr1 and ERK1/2 activation for V₂R^T360A. Molecular dynamics simulations reveal that Ib30 enriches active-like βarr1 conformation with respect to the inter-domain rotation, and cellular assays demonstrate that it also enhances βarr1-β₂-adaptin interaction. Our data provide an experimental framework to positively modulate the receptor-transducer-effector axis for GPCRs using intrabodies, which can be potentially integrated in the paradigm of GPCR-targeted drug discovery.

G protein-coupled receptors (GPCRs) are integral membrane proteins and the largest class of drug targets in the human genome. Here, Baidya et al. show that a synthetic antibody can be used to modulate GPCR trafficking and signaling in live cells.

Emerging structural insights into GPCR-β-arrestin interaction and functional outcomes

Agonist-induced recruitment of β-arrestins (βarrs) to G protein-coupled receptors (GPCRs) plays a central role in regulating the spatio-temporal aspects of GPCR signaling. Several recent studies have provided novel structural and functional insights into our understanding of GPCR-βarr interaction, subsequent βarr activation and resulting functional outcomes. In this review, we discuss these recent advances with a particular emphasis on recognition of receptor-bound phosphates by βarrs, the emerging concept of spatial positioning of key phosphorylation sites, the conformational transition in βarrs during partial to full-engagement, and structural differences driving functional outcomes of βarr isoforms. We also highlight the key directions that require further investigation going forward to fully understand the structural mechanisms driving βarr activation and functional responses.

An intrabody sensor to monitor conformational activation of β-arrestins

Agonist-induced interaction of β-arrestins with GPCRs is critically involved in downstream signaling and regulation. This interaction is associated with activation and major conformational changes in β-arrestins. Although there are some assays available to monitor the conformational changes in β-arrestins in cellular context, additional sensors to report β-arrestin activation, preferably with high-throughput capability, are likely to be useful considering the structural and functional diversity in GPCR-β-arrestin complexes. We have recently developed an intrabody-based sensor as an integrated approach to monitor GPCR-β-arrestin interaction and conformational change, and generated a luminescence-based reporter using NanoBiT complementation technology. This sensor is derived from a synthetic antibody fragment referred to as Fab30 that selectively recognizes activated and receptor-bound conformation of β-arrestin1. Here, we present a step-by-step protocol to employ this intrabody sensor to measure the interaction and conformational activation of β-arrestin1 upon agonist-stimulation of a prototypical GPCR, the complement C5a receptor (C5aR1). This protocol is potentially applicable to other GPCRs and may also be leveraged to deduce qualitative differences in β-arrestin1 conformations induced by different ligands and receptor mutants.

Comparative Immunogenicity of Bacterially Expressed Soluble Trimers and Nanoparticle Displayed Influenza Hemagglutinin Stem Immunogens

Current influenza vaccines need to be updated annually due to mutations in the globular head of the viral surface protein, hemagglutinin (HA). To address this, vaccine candidates have been designed based on the relatively conserved HA stem domain and have shown protective efficacy in animal models. Oligomerization of the antigens either by fusion to oligomerization motifs or display on self-assembling nanoparticle scaffolds, can induce more potent immune responses compared to the corresponding monomeric antigen due to multivalent engagement of B-cells. Since nanoparticle display can increase manufacturing complexity, and often involves one or more mammalian cell expressed components, it is important to characterize and compare various display and oligomerization scaffolds. Using a structure guided approach, we successfully displayed multiple copies of a previously designed soluble, trimeric influenza stem domain immunogen, pH1HA10, on the ferritin like protein, MsDps2 (12 copies), Ferritin (24 copies) and Encapsulin (180 copies). All proteins were expressed in Escherichia coli. The nanoparticle fusion immunogens were found to be well folded and bound to the influenza stem directed broadly neutralizing antibodies with high affinity. An 8.5 Å Cryo-EM map of Msdps2-pH1HA10 confirmed the successful design of the nanoparticle fusion immunogen. Mice immunization studies with the soluble trimeric stem and nanoparticle fusion constructs revealed that all of them were immunogenic, and protected mice against homologous (A/Belgium/145-MA/2009) and heterologous (A/Puerto Rico/8/1934) challenge with 10MLD50 mouse adapted virus. Although nanoparticle display conferred a small but statistically significant improvement in protection relative to the soluble trimer in a homologous challenge, heterologous protection was similar in both nanoparticle-stem immunized and trimeric stem immunized groups. Such rapidly producible, bacterially expressed antigens and nanoparticle scaffolds are useful modalities to tackle future influenza pandemics.

Making the switch: The role of Gq in driving GRK selectivity at GPCRs

Selective engagement of signal transducers such as G proteins and β-arrestins with GPCRs upon stimulation with biased agonists is thought to be due to distinct receptor conformations. Kawakami et al. propose an additional mechanism whereby activation of Gq determines GPCR kinase subtype selectivity to the activated angiotensin receptor, leading to distinct binding modalities of β-arrestins and functional outcomes.

Community Guidelines for GPCR Ligand Bias: IUPHAR Review XX

G protein-coupled receptors modulate a plethora of physiological processes and mediate the effects of one-third of FDA-approved drugs. Depending on which ligand activates a receptor, it can engage different intracellular transducers. This 'biased signaling' paradigm requires that we now characterize physiological signaling not just by receptors but by ligand-receptor pairs. Ligands eliciting biased signaling may constitute better drugs with higher efficacy and fewer adverse effects. However, ligand bias is very complex, making reproducibility and description challenging. Here, we provide guidelines and terminology for any scientists to design and report ligand bias experiments. The guidelines will aid consistency and clarity, as the basic receptor research and drug discovery communities continue to advance our understanding and exploitation of ligand bias. Scientific insight, biosensors, and analytical methods are still evolving and should benefit from and contribute to the implementation of the guidelines, together improving translation from in vitro to disease-relevant in vivo models.

Biphasic activation of β-arrestin 1 upon interaction with a GPCR revealed by methyl-TROSY NMR

β-arrestins (βarrs) play multifaceted roles in the function of G protein-coupled receptors (GPCRs). βarrs typically interact with phosphorylated C-terminal tail (C tail) and transmembrane core (TM core) of GPCRs. However, the effects of the C tail- and TM core-mediated interactions on the conformational activation of βarrs have remained elusive. Here, we show the conformational changes for βarr activation upon the C tail- and TM core-mediated interactions with a prototypical GPCR by nuclear magnetic resonance (NMR) spectroscopy. Our NMR analyses demonstrated that while the C tail-mediated interaction alone induces partial activation, in which βarr exists in equilibrium between basal and activated conformations, the TM core- and the C tail-mediated interactions together completely shift the equilibrium toward the activated conformation. The conformation-selective antibody, Fab30, promotes partially activated βarr into the activated-like conformation. This plasticity of βarr conformation in complex with GPCRs engaged in different binding modes may explain the multifunctionality of βarrs.

β-arrestins commonly bind to two distinct elements in GPCRs: the phosphorylated carboxyl terminal tail (C tail) and the cytoplasmic face of the transmembrane region (TM core). Here, the authors use methyl-TROSY NMR measurements to characterise the interactions between β-arrestin 1 (βarr1) and a GPCR and observe that C tail-mediated interaction with a GPCR alone induces the partial activation of βarr1, whereas the TM core- and C tail-mediated interactions together stabilize the activated conformation of βarr1.

Intrinsic bias at non-canonical, β-arrestin-coupled seven transmembrane receptors

G-protein-coupled receptors (GPCRs), also known as seven transmembrane receptors (7TMRs), typically interact with two distinct signal-transducers, i.e., G proteins and β-arrestins (βarrs). Interestingly, there are some non-canonical 7TMRs that lack G protein coupling but interact with barrs, although an understanding of their transducer coupling preference, downstream signaling, and structural mechanism remains elusive. Here, we characterize two such non-canonical 7TMRs, namely, the decoy D6 receptor (D6R) and the complement C5a receptor subtype 2 (C5aR2), in parallel with their canonical GPCR counterparts. We discover that D6R and C5aR2 efficiently couple to βarrs, exhibit distinct engagement of GPCR kinases (GRKs), and activate non-canonical downstream signaling pathways. We also observe that βarrs adopt distinct conformations for D6R and C5aR2, compared to their canonical GPCR counterparts, in response to common natural agonists. Our study establishes D6R and C5aR2 as βarr-coupled 7TMRs and provides key insights into their regulation and signaling with direct implication for biased agonism.

The use of fluorescence correlation spectroscopy to monitor cell surface β2-adrenoceptors at low expression levels in human embryonic stem cell-derived cardiomyocytes and fibroblasts

The importance of cell phenotype in determining the molecular mechanisms underlying β₂ -adrenoceptor (β2AR) function has been noted previously when comparing responses in primary cells and recombinant model cell lines. Here, we have generated haplotype-specific SNAP-tagged β2AR human embryonic stem (ES) cell lines and applied fluorescence correlation spectroscopy (FCS) to study cell surface receptors in progenitor cells and in differentiated fibroblasts and cardiomyocytes. FCS was able to quantify SNAP-tagged β2AR number and diffusion in both ES-derived cardiomyocytes and CRISPR/Cas9 genome-edited HEK293T cells, where the expression level was too low to detect using standard confocal microscopy. These studies demonstrate the power of FCS in investigating cell surface β2ARs at the very low expression levels often seen in endogenously expressing cells. Furthermore, the use of ES cell technology in combination with FCS allowed us to demonstrate that cell surface β2ARs internalize in response to formoterol-stimulation in ES progenitor cells but not following their differentiation into ES-derived fibroblasts. This indicates that the process of agonist-induced receptor internalization is strongly influenced by cell phenotype and this may have important implications for drug treatment with long-acting β2AR agonists.

Dissecting the structural features of β-arrestins as multifunctional proteins

β-arrestins bind active G protein-coupled receptors (GPCRs) and play a crucial role in receptor desensitization and internalization. The classical paradigm of arrestin function has been expanded with the identification of many non-receptor-binding partners, which indicated the multifunctional role of β-arrestins in cellular functions. To elucidate the molecular mechanism of β-arrestin-mediated signaling, the structural features of β-arrestins were investigated using X-ray crystallography and cryogenic electron microscopy (cryo-EM). However, the intrinsic conformational flexibility of β-arrestins hampers the elucidation of structural interactions between β-arrestins and their binding partners using conventional structure determination tools. Therefore, structural information obtained using complementary structure analysis techniques would be necessary in combination with X-ray crystallography and cryo-EM data. In this review, we describe how β-arrestins interact with their binding partners from a structural point of view, as elucidated by both traditional methods (X-ray crystallography and cryo-EM) and complementary structure analysis techniques.

Biased agonism at chemokine receptors

In the human chemokine system, interactions between the approximately 50 known endogenous chemokine ligands and 20 known chemokine receptors (CKRs) regulate a wide range of cellular functions and biological processes including immune cell activation and homeostasis, development, angiogenesis, and neuromodulation. CKRs are a family of G protein-coupled receptors (GPCR), which represent the most common and versatile class of receptors in the human genome and the targets of approximately one third of all Food and Drug Administration-approved drugs. Chemokines and CKRs bind with significant promiscuity, as most CKRs can be activated by multiple chemokines and most chemokines can activate multiple CKRs. While these ligand-receptor interactions were previously regarded as redundant, it is now appreciated that many chemokine:CKR interactions display biased agonism, the phenomenon in which different ligands binding to the same receptor signal through different pathways with different efficacies, leading to distinct biological effects. Notably, these biased responses can be modulated through changes in ligand, receptor, and or the specific cellular context (system). In this review, we explore the biochemical mechanisms, functional consequences, and therapeutic potential of biased agonism in the chemokine system. An enhanced understanding of biased agonism in the chemokine system may prove transformative in the understanding of the mechanisms and consequences of biased signaling across all GPCR subtypes and aid in the development of biased pharmaceuticals with increased therapeutic efficacy and safer side effect profiles.

DeSiphering receptor core-induced and ligand-dependent conformational changes in arrestin via genetic encoded trimethylsilyl 1H-NMR probe

Characterization of the dynamic conformational changes in membrane protein signaling complexes by nuclear magnetic resonance (NMR) spectroscopy remains challenging. Here we report the site-specific incorporation of 4-trimethylsilyl phenylalanine (TMSiPhe) into proteins, through genetic code expansion. Crystallographic analysis revealed structural changes that reshaped the TMSiPhe-specific amino-acyl tRNA synthetase active site to selectively accommodate the trimethylsilyl (TMSi) group. The unique up-field 1H-NMR chemical shift and the highly efficient incorporation of TMSiPhe enabled the characterization of multiple conformational states of a phospho-β2 adrenergic receptor/β-arrestin-1(β-arr1) membrane protein signaling complex, using only 5 μM protein and 20 min of spectrum accumulation time. We further showed that extracellular ligands induced conformational changes located in the polar core or ERK interaction site of β-arr1 via direct receptor transmembrane core interactions. These observations provided direct delineation and key mechanism insights that multiple receptor ligands were able to induce distinct functionally relevant conformational changes of arrestin.

Distinct phosphorylation sites in a prototypical GPCR differently orchestrate β-arrestin interaction, trafficking, and signaling

Agonist-induced phosphorylation of G protein-coupled receptors (GPCRs) is a key determinant for their interaction with β-arrestins (βarrs) and subsequent functional responses. Therefore, it is important to decipher the contribution and interplay of different receptor phosphorylation sites in governing βarr interaction and functional outcomes. Here, we find that several phosphorylation sites in the human vasopressin receptor (V₂R), positioned either individually or in clusters, differentially contribute to βarr recruitment, trafficking, and ERK1/2 activation. Even a single phosphorylation site in V₂R, suitably positioned to cross-talk with a key residue in βarrs, has a decisive contribution in βarr recruitment, and its mutation results in strong G-protein bias. Molecular dynamics simulation provides mechanistic insights into the pivotal role of this key phosphorylation site in governing the stability of βarr interaction and regulating the interdomain rotation in βarrs. Our findings uncover important structural aspects to better understand the framework of GPCR-βarr interaction and biased signaling.

Key phosphorylation sites in GPCRs orchestrate the contribution of β-Arrestin 1 in ERK1/2 activation

β-arrestins (βarrs) are key regulators of G protein-coupled receptor (GPCR) signaling and trafficking, and their knockdown typically leads to a decrease in agonist-induced ERK1/2 MAP kinase activation. Interestingly, for some GPCRs, knockdown of βarr1 augments agonist-induced ERK1/2 phosphorylation although a mechanistic basis for this intriguing phenomenon is unclear. Here, we use selected GPCRs to explore a possible correlation between the spatial positioning of receptor phosphorylation sites and the contribution of βarr1 in ERK1/2 activation. We discover that engineering a spatially positioned double-phosphorylation-site cluster in the bradykinin receptor (B₂ R), analogous to that present in the vasopressin receptor (V₂ R), reverses the contribution of βarr1 in ERK1/2 activation from inhibitory to promotive. An intrabody sensor suggests a conformational mechanism for this role reversal of βarr1, and molecular dynamics simulation reveals a bifurcated salt bridge between this double-phosphorylation site cluster and Lys²⁹⁴ in the lariat loop of βarr1, which directs the orientation of the lariat loop. Our findings provide novel insights into the opposite roles of βarr1 in ERK1/2 activation for different GPCRs with a direct relevance to biased agonism and novel therapeutics.

Crystal Structure of β-Arrestin 2 in Complex with CXCR7 Phosphopeptide

β-Arrestins (βarrs) critically regulate G-protein-coupled receptor (GPCR) signaling and trafficking. βarrs have two isoforms, βarr1 and βarr2. Receptor phosphorylation is a key determinant for the binding of βarrs, and understanding the intricate details of receptor-βarr interaction is the next frontier in GPCR structural biology. The high-resolution structure of active βarr1 in complex with a phosphopeptide derived from GPCR has been revealed, but that of βarr2 remains elusive. Here, we present a 2.3-Å crystal structure of βarr2 in complex with a phosphopeptide (C7pp) derived from the carboxyl terminus of CXCR7. The structural analysis of C7pp-bound βarr2 reveals key differences from the previously determined active conformation of βarr1. One of the key differences is that C7pp-bound βarr2 shows a relatively small inter-domain rotation. Antibody-fragment-based conformational sensor and hydrogen/deuterium exchange experiments further corroborated the structural features of βarr2 and suggested that βarr2 adopts a range of inter-domain rotations.

Molecular basis of β-arrestin coupling to formoterol-bound β1-adrenoceptor

The β₁-adrenoceptor (β₁AR) is a G-protein-coupled receptor (GPCR) that couples¹ to the heterotrimeric G protein G_s. G-protein-mediated signalling is terminated by phosphorylation of the C terminus of the receptor by GPCR kinases (GRKs) and by coupling of β-arrestin 1 (βarr1, also known as arrestin 2), which displaces G_s and induces signalling through the MAP kinase pathway². The ability of synthetic agonists to induce signalling preferentially through either G proteins or arrestins-known as biased agonism³-is important in drug development, because the therapeutic effect may arise from only one signalling cascade, whereas the other pathway may mediate undesirable side effects⁴. To understand the molecular basis for arrestin coupling, here we determined the cryo-electron microscopy structure of the β₁AR-βarr1 complex in lipid nanodiscs bound to the biased agonist formoterol⁵, and the crystal structure of formoterol-bound β₁AR coupled to the G-protein-mimetic nanobody⁶ Nb80. βarr1 couples to β₁AR in a manner distinct to that⁷ of G_s coupling to β₂AR-the finger loop of βarr1 occupies a narrower cleft on the intracellular surface, and is closer to transmembrane helix H7 of the receptor when compared with the C-terminal α5 helix of G_s. The conformation of the finger loop in βarr1 is different from that adopted by the finger loop of visual arrestin when it couples to rhodopsin⁸. β₁AR coupled to βarr1 shows considerable differences in structure compared with β₁AR coupled to Nb80, including an inward movement of extracellular loop 3 and the cytoplasmic ends of H5 and H6. We observe weakened interactions between formoterol and two serine residues in H5 at the orthosteric binding site of β₁AR, and find that formoterol has a lower affinity for the β₁AR-βarr1 complex than for the β₁AR-G_s complex. The structural differences between these complexes of β₁AR provide a foundation for the design of small molecules that could bias signalling in the β-adrenoceptors.

Genetically encoded intrabody sensors report the interaction and trafficking of β-arrestin 1 upon activation of G-protein-coupled receptors

Agonist stimulation of G-protein-coupled receptors (GPCRs) typically leads to phosphorylation of GPCRs and binding to multifunctional proteins called β-arrestins (βarrs). The GPCR-βarr interaction critically contributes to GPCR desensitization, endocytosis, and downstream signaling, and GPCR-βarr complex formation can be used as a generic readout of GPCR and βarr activation. Although several methods are currently available to monitor GPCR-βarr interactions, additional sensors to visualize them may expand the toolbox and complement existing methods. We have previously described antibody fragments (FABs) that recognize activated βarr1 upon its interaction with the vasopressin V2 receptor C-terminal phosphopeptide (V2Rpp). Here, we demonstrate that these FABs efficiently report the formation of a GPCR-βarr1 complex for a broad set of chimeric GPCRs harboring the V2R C terminus. We adapted these FABs to an intrabody format by converting them to single-chain variable fragments and used them to monitor the localization and trafficking of βarr1 in live cells. We observed that upon agonist simulation of cells expressing chimeric GPCRs, these intrabodies first translocate to the cell surface, followed by trafficking into intracellular vesicles. The translocation pattern of intrabodies mirrored that of βarr1, and the intrabodies co-localized with βarr1 at the cell surface and in intracellular vesicles. Interestingly, we discovered that intrabody sensors can also report βarr1 recruitment and trafficking for several unmodified GPCRs. Our characterization of intrabody sensors for βarr1 recruitment and trafficking expands currently available approaches to visualize GPCR-βarr1 binding, which may help decipher additional aspects of GPCR signaling and regulation.

Differential Role of Serines and Threonines in Intracellular Loop 3 and C-Terminal Tail of the Histamine H4 Receptor in β-Arrestin and G Protein-Coupled Receptor Kinase Interaction, Internalization, and Signaling

The histamine H₄ receptor (H₄R) activates Gα_i-mediated signaling and recruits β-arrestin2 upon stimulation with histamine. β-Arrestins play a regulatory role in G protein-coupled receptor (GPCR) signaling by interacting with phosphorylated serine and threonine residues in the GPCR C-terminal tail and intracellular loop 3, resulting in receptor desensitization and internalization. Using bioluminescence resonance energy transfer (BRET)-based biosensors, we show that G protein-coupled receptor kinases (GRK) 2 and 3 are more quickly recruited to the H₄R than β-arrestin1 and 2 upon agonist stimulation, whereas receptor internalization dynamics toward early endosomes was slower. Alanine-substitution revealed that a serine cluster at the distal end of the H₄R C-terminal tail is essential for the recruitment of β-arrestin1/2, and consequently, receptor internalization and desensitization of G protein-driven extracellular-signal-regulated kinase (ERK)1/2 phosphorylation and label-free cellular impedance. In contrast, alanine substitution of serines and threonines in the intracellular loop 3 of the H₄R did not affect β-arrestin2 recruitment and receptor desensitization, but reduced β-arrestin1 recruitment and internalization. Hence, β-arrestin recruitment to H₄R requires the putative phosphorylated serine cluster in the H₄R C-terminal tail, whereas putative phosphosites in the intracellular loop 3 have different effects on β-arrestin1 versus β-arrestin2. Mutation of these putative phosphosites in either intracellular loop 3 or the C-terminal tail did not affect the histamine-induced recruitment of GRK2 and GRK3 but does change the interaction of H₄R with GRK5 and GRK6, respectively. Identification of H₄R interactions with these proteins is a first step in the understanding how this receptor might be dysregulated in pathophysiological conditions.

Terminating G-Protein Coupling: Structural Snapshots of GPCR-β-Arrestin Complexes

β-arrestins (βarrs) play multifaceted roles in the signaling and regulation of G-protein-coupled receptors (GPCRs) including their desensitization and endocytosis. Recently determined cryo-EM structures of two different GPCRs in complex with βarr1 provide the first glimpse of GPCR-βarr engagement and a structural framework to understand their interaction.

Structure and function of β-arrestins, their emerging role in breast cancer, and potential opportunities for therapeutic manipulation

β-Arrestins (βarrs) are multifunctional intracellular proteins with an ability to directly interact with a large number of cellular partners including the G protein-coupled receptors (GPCRs). βarrs contribute to multiple aspects of GPCR signaling, trafficking and downregulation. Considering the central involvement of GPCR signaling in the onset and progression of diverse types of cancers, βarrs have also emerged as key players in the context of investigating cancer phenotypes, and as potential therapeutic targets. In this chapter, we first provide a brief account of structure and function of βarrs and then highlight recent discoveries unfolding novel functional attributes of βarrs in breast cancer. We also underscore the recent paradigms of modulating βarr functions in cellular context and potential therapeutic opportunities going forward.

Site-directed labeling of β-arrestin with monobromobimane for measuring their interaction with G protein-coupled receptors

β-arrestins (βarrs) are multifunctional proteins that interact with activated and phosphorylated G protein-coupled receptors (GPCRs) to regulate their signaling and trafficking. Understanding the intricate details of GPCR-βarr interaction continues to be a key research area in the field of GPCR biology. Bimane fluorescence spectroscopy has been one of the key approaches among a broad range of methods employed to study GPCR-βarr interaction using purified and reconstituted system. Here, we present a step-by-step protocol for labeling βarrs with monobromobimane (mBBr) in a site-directed fashion for measuring their interaction with GPCRs and the resulting conformational changes. This simple protocol can be directly applied to other protein-protein interaction modules as well for measuring interactions and conformational changes in reconstituted systems in vitro.

I8-arachnotocin-an arthropod-derived G protein-biased ligand of the human vasopressin V2 receptor

The neuropeptides oxytocin (OT) and vasopressin (VP) and their G protein-coupled receptors OTR, V_1aR, V_1bR, and V₂R form an important and widely-distributed neuroendocrine signaling system. In mammals, this signaling system regulates water homeostasis, blood pressure, reproduction, as well as social behaviors such as pair bonding, trust and aggression. There exists high demand for ligands with differing pharmacological profiles to study the physiological and pathological functions of the individual receptor subtypes. Here, we present the pharmacological characterization of an arthropod (Metaseiulus occidentalis) OT/VP-like nonapeptide across the human OT/VP receptors. I8-arachnotocin is a full agonist with respect to second messenger signaling at human V₂R (EC₅₀ 34 nM) and V_1bR (EC₅₀ 1.2 µM), a partial agonist at OTR (EC₅₀ 790 nM), and a competitive antagonist at V_1aR [pA₂ 6.25 (558 nM)]. Intriguingly, I8-arachnotocin activated the Gα_s pathway of V₂R without recruiting either β-arrestin-1 or β-arrestin-2. I8-arachnotocin might thus be a novel pharmacological tool to study the (patho)physiological relevance of β-arrestin-1 or -2 recruitment to the V₂R. These findings furthermore highlight arthropods as a novel, vast and untapped source for the discovery of novel pharmacological probes and potential drug leads targeting neurohormone receptors.

Reversible biotinylation of purified proteins for measuring protein-protein interactions

Measuring protein-protein interactions using purified proteins in vitro is one of the most frequently used approach to understand the biochemical and mechanistic details of cellular signaling pathways. Typically, affinity tags are genetically fused to proteins of interest, and they are used to capture and detect them. However, in some cases, fusion of bulky affinity tags might present a significant limitation in these experiments, especially if the regions in close proximity of tags are involved in protein-protein interactions. Here, we present a step-by-step protocol for an alternative approach that involves reversible biotinylation of purified proteins using a simple chemical-conjugation of cleavable biotin moiety. Biotinylated proteins can be directly used as bait for selective immobilization on solid support for measuring protein-protein interactions. Furthermore, biotinylation of protein of interest also allows specific detection in standard biochemical assays. This simple, straightforward and modular protocol can be directly adapted and applied to facilitate the detection of novel protein-protein interactions as well as measuring apparent affinities of such interactions.